The invention relates to a voltage controlled oscillator used in a phase locked loop or the like.
Recently, the phase locked loop (PLL) has been widely used, although it is complex in construction. This tendency is due largely to recent remarkable development of integrated circuit technology. The PLL mainly comprises a programmable counter, a reference frequency divider, a phase comparator, a voltage controlled oscillator (VCO) and the like. These circuit components except the VCO are constructed by large-scale integrating complementary metal oxide semiconductor field effect transistors (CMOS FET's) on a single semiconductor chip. Many kinds of such type circuit components have been developed and are commercially available. VCO's in PLL's typically comprise an LC oscillator using a variable capacitance diode and an emitter follower multivibrator. However, such VCO's are difficult to fabricate using LSI technology, although a few VCO's using LSI of CMOS FET's have been developed. The VCO's of such a type are operable in a frequency range up to at most 2 MHz. This restricts their applications to within a narrow scope.
One form of CMOS LSI constructed oscillators of frequency fixed type as shown in FIG. 1 has been proposed of which some elaboration will be given below. Reference numerals 12, 14 and 16 are inverters of CMOS construction being connected in cascade fashion. A resistor 18 is connected between the output side of the inverter 16 and the input side of the inverter 12. The resistor 18 provides a path of positive feedback from the output of the inverter 16 to the input thereof. Between the input of the inverter 16 and the output of the inverter 14, a capacitor 20 is coupled. The product of the capacitance C.sub.1 of capacitor 20 and the resistance R.sub.1 of the resistor 18 gives the time constant of the circuit. The oscillating period T of the oscillator circuit is given: ##EQU1## where V.sub.DD is a power source voltage, and V.sub.TH is a threshold voltage of the inverter 16.
FIGS. 2A, 2B and 2C show signal waveforms at the portions (a), (b) and (c) in the FIG. 1 oscillator, respectively.
From the equation (1), it will be seen that the period T may be changed by changing the resistance R.sub.1 while fixing the capacitance C.sub.1.
This fact leads to the invention.